1. Field of the Invention
The present invention relates to an image reading apparatus for reading an image of a document.
2. Description of the Related Art
FIG. 9 illustrates a conventional image reading apparatus. In the conventional image reading apparatus, a document D is fed onto a platen glass 261 by a read roller pair 232, and the document is discharged from the platen glass 261 by a read discharge roller pair 233. An image of the document D is read by a reading unit 260 which is remained still at a position directly below the platen glass 261.
As illustrated in FIG. 9, the conventional image reading apparatus is equipped with a press contact releasing cam mechanism which gradually releases pressure to a read driven roller 214 of the read roller pair 232, and a control unit which controls operations of the press contact releasing cam mechanism before an upstream end portion of the document D passes the read roller pair 232. Due to this construction, the conveyance of the document D is stabilized (See Japanese Patent Application Laid-Open No. 2003-34450).
In an image reading apparatus which does not include the above described press contact releasing cam mechanism, when the document D passes through the read roller pair arranged on the upstream side of the reading unit and is released from the pinching by the read roller pair, the document may be vibrated or there may be generated a minute fluctuation in the document speed. If such vibration of the upstream end portion of the document D or a fluctuation in speed is generated, the conventional image reading apparatus tries to read the image of the document D while it is vibrating, so that it is rather difficult to obtain a read image with a satisfactory quality.
In view of this, in the conventional image reading apparatus, the pressure on the read driven roller 214 is gradually released before the document passes the read roller pair 232, so that disturbance of the read image due to the influence of the vibration generated when the document D passes through the read roller pair 232 is prevented.
The conventional image reading apparatus adopts an effective construction in which no vibration of the upstream end portion of the document is generated by releasing the pressure on the read driven roller 214 of the read roller pair 232. However, in the conventional image reading apparatus, there is provided a dedicated drive unit for releasing pressure, resulting in a rather complicated mechanism and high cost.
Japanese Patent Application Laid-Open No. 2003-189069 discusses a configuration provided with a plurality of platen rollers facing to the platen glass. However, in the configuration discussed in Japanese Patent Application Laid-Open No. 2003-189069, no measures are taken against the vibration of the document and the minute fluctuation in document speed when the document is released from the pinching by the read roller pair provided on the upstream side of the platen glass with respect to the conveyance direction.